Consider the question, ``Who is a grandparent of Harmonia?''
This can be expressed as &exist x Grandparent(x, Harmonia) ; this is
negated to be &forall x &not Grandparent(x, Harmonia) (``Nobody is
the grandparent of Harmonia.'').

We then can use resolution to derive □, thus proving that
Harmonia does have a grandparent. But who is it?